The present invention relates to telecommunications devices and, in particular, to an improved automatic power control loop for a wireless telecommunications transmitter.
The Federal Communications Commission (FCC) regulates the use of the radio frequency (RF) spectrum in the United States. Users of allocated bandwidth of the RF spectrum must take measures to ensure that radiated emissions inside and outside the allocated bandwidth are maintained within acceptable levels to avoid interfering with other users' operating in the same or other bandwidths. For example, users of cellular telephone systems must ensure that they are compliant with the level of radiated emissions allowable inside or outside the channels they have been assigned.
Cellular telephones use variable power control to adjust the output power to the requirements of the system specification and additionally to limit the highest output power level to minimize specific absorption rates (SAR) and out of channel radiation such as adjacent channel power ratio (ACPR) and spurious emissions.
Components in the transmit chain, and particularly the power amplifier, tend to have a relatively wide variation in gain with respect to unit, temperature and frequency. To maintain a given output power over all units, temperature, and frequency would typically require a multidimensional calibration table. Some phone manufacturers use an expensive power detection circuit that allows closed loop power control over the entire power range of the phone.
On time division multiple access (TDMA) phones, transmit power must conform to TIA/EIA specification IS136-270. This specification details ten power levels and four mobile classes. A class IV mobile must unit transmit the power detailed in the following table:
POWERLEVELOUTPUT POWER028dBm128dBm228dBm324dBm420dBm516dBm612dBm78dBm84dBm90dBm10 −4dBmPower Levels 0 through 7 must be accurate to plus or minus 3 dB, and Power Levels 8, 9 and 10 can be less accurate.